Two Types Of Simulation And Disappearance Of Electromagnetic Waves In A Rectangular Waveguide

When the frequency is below than the cut-off frequency of the waveguide, the field intensity decays exponentially with distance along the waveguide and the wave is called evanescent wave. Some experiments have been carried out successfully using the evanescent waveguide by European scientists, thus, the rang of exploitation of waveguides has been extended widely. Because of the complexity and particularity of evanescent waves, a pure theoretical treatment can only provide a qualitative description, a well-developed 3D EM code ,SOS6 which is based on the MAGIC Command Language with the finite difference time domain method, is used to simulating the EM evanescent waves in this article. The followings were discovered in the simulations of undersized waveguide:When the pulse width of input signal , a Gaussian packet, is rather wide, the output signal distorted little. A superluminal group velocity 3.21c was revealed when the wave packet crossing 50mm forbidden region. And the intensity of electric field decayed by -41.5dB.When the length of forbidden region was increased, the output signal distorted seriously and some obvious high frequency components were found. The author thought that the high frequency components were included in the input signal originally. After having crossed the forbidden region, they have not been decayed but the main frequencies in the input signal have been decayed enormously. When the pulse width of input Gaussian wave packet was reduced and the length of forbidden region retained, the result displayed the output signal distorted seriously and its spectrum changed very hard. A frequency above the cut-off frequency became the main frequency and the group velocity was below than c. It was the result from that the width of spectrum of the input EM pulse packet was too wide, the spectrum included rather strong frequencies being above the cut-off frequency which have not been decayed. The simulations of heterostructure wavegiude indicated that the group velocity of the wave packet was superluminal and the intensity of electric field decayed by -42dB.